This proposal addresses a fundamental issue in neuroscience, namely, how do sensory stimuli acquire emotional significance? More specifically, how are fearful responses to stimuli extinguished once the stimuli no longer predict danger? The acquisition of fear associations to aversive stimuli occurs through a form of classical conditioning known as fear conditioning. In auditory fear conditioning, a tone conditioned stimulus (CS) is paired with a footshock unconditioned stimulus (US), resulting in the acquisition of fear responses to the tone such as freezing and response suppression. Fear conditioning depends critically on the amygdala. At present, the neural circuits of extinction are unknown. Previous studies suggest that the ventral medial prefrontal cortex (vmPFC), which projects to the amygdala, is necessary for consolidation of extinction learning. Blockade of NMDA glutamate receptors, which are involved in synaptic plasticity, prevents extinction. The central hypothesis of this proposal is that extinction learning requires NMDA-mediated plasticity in prefrontal-amygdala circuits. Three experiments will test this hypothesis: 1) NMDA receptor blockers will be microinjected directly into the vmPFC or its targets in the amygdala during extinction (hypothesis: NMDA blockade of vmPFC targets in the amygdala will not block the expression of extinction, but will prevent consolidation of extinction). 2) The effect of NMDA blockade on extinction-induced firing correlates of vmPFC neurons will be tested (hypothesis: NMDA blockade will prevent extinction-induced firing correlates in vmPFC, and cause increased recovery of extinguished fear). 3) Changes in the expression of immediate early genes during extinction will be measured (hypothesis: extinction will induce changes in gene expression in the vmPFC and/or the central nucleus of the amygdala that correlate with the amount of conditioned fear recovered after 24 hours). This proposal will combine behavioral, cellular and molecular approaches in order to understand extinction circuits. Deficits in extinction learning may underlie anxiety disorders such as post-traumatic stress disorder and specific phobia. This research will advance our understanding of fear processes in the brain, and will ultimately lead to more effective treatments for these disorders.